Recently, a developing device using a two-component developer including a toner and a magnetic carrier is mainly used for image forming apparatuses such as copiers, printers, facsimiles and multifunction products having two or more functions of a copier function, a printer function, and a facsimile function. A developing device illustrated in FIG. 4 is an example of such a developing device.
Referring to FIG. 4, a developing device 100 includes a developing sleeve 101 serving as a developer bearing member, a first feeding screw 102 and a second feeding screw 103, which feed a two-component developer to the developing sleeve 101 while circulating the developer, and a first developer containing part 104 and a second developer containing part 105, which contain the two-component developer including a toner and a magnetic carrier. In this regard, central portions of the first and second developer containing parts 104 and 105 are separated from each other by a partition 106 provided on a casing of the developing device 100, and communication holes 107 and 108 are formed on both end portions of the first and second developer containing parts 104 and 105 to communicate the developer containing parts 104 and 105 with each other.
A drive motor 109 is provided on a shaft of the developing sleeve 101 to directly transfer a rotation driving force of the drive motor 109 to the shaft of the developing sleeve 101, thereby rotating the developing sleeve 101. An axle gear 110 provided on the shaft of the developing sleeve 101 is engaged with a first receiver gear 111 provided on one end portion of the first feeding screw 102. Since the gears 110 and 111 are engaged with each other, the rotation driving force of the drive motor 109 is transmitted to the first feeding screw 102, thereby rotating the first feeding screw 102. In addition, an intermediate gear 112 is provided on the other end portion of the first feeding screw 102 so as to be engaged with a second receiver gear 113. Since the gears 112 and 113 are engaged with each other, the rotation driving force of the drive motor 109 is transmitted to the second feeding screw 103, thereby rotating the second feeding screw 103.
The two-component developer (hereinafter referred to as a developer) contained in the second developer containing part 105 is fed from left to right in FIG. 4 by the second feeding screw 103. The developer thus fed by the second feeding screw 103 is then fed to the first developer containing part 104 through the right communication hole 107. The first feeding screw 102, which is arranged in the vicinity of the developing sleeve 101, feeds the developer in the first developer containing part 104 from right to left in FIG. 4 while feeding the developer to the developing sleeve 101. In addition, the first feeding screw 102 feeds the developer in the first developer containing part 104 from right to left in FIG. 4 while receiving the developer, which has been used for developing an electrostatic latent image on a photoreceptor drum 1, from the developing sleeve 101. The developer fed to the left end of the first developer containing part 104 by the first feeding screw 102 is returned to the second developer containing part 105 through the communication hole 108. Thus, the developer in the developing device 100 is circulated in the first and second developer containing parts 104 and 105.
The developing sleeve 101 bears thereon the developer fed by the first feeding screw 102 by means of a magnetic force of a magnet roller provided in the developing sleeve 101. An electrostatic latent image formed on the photoreceptor 1 serving as a latent image bearer is developed with the toner included in the developer on the developing sleeve 101. The developer used for development, in which the toner therein is consumed for development, is returned to the first developer containing part 104 to be mixed with the developer fed by the first feeding screw 102. The mixed developer is fed to the second developer containing part 105, and is then mixed with a toner supplied from a toner supply opening 114 to increase the toner concentration of the developer. In this case, a magnetic permeability sensor to detect the toner concentration of the developer is provided on a proper portion of the second developer containing part 105 to determine the mixing ratio of the magnetic carrier to the toner from the measured magnetic permeability. A toner supplying device supplies the toner to the second developer containing part 105 of the developing device 100 if desired so that the developer mixed with the toner has a toner concentration in the predetermined concentration range.
Among image forming apparatus using such a developing device 100, an image forming apparatus is known which has multiple print modes and changes the process linear velocity thereof when a user changes the print mode. In this regard, for example, change of the process linear velocity is performed as follows. Specifically, when a standard mode is selected from the multiple print modes using an operation part, image forming members such as the photoreceptor 1 and the developing sleeve 101 are rotated at a predetermined reference linear velocity. When an image quality oriented mode is selected, the image forming members are rotated at a linear velocity lower than the reference linear velocity. In contrast, when a print speed oriented mode is selected, the image forming members are rotated at a linear velocity higher than the reference linear velocity.
There is a proposal in which the developer feeding speed of the first developer feeding screw is controlled independently of that of the second developer feeding screw. Specifically, the developer feeding speed of the second developer feeding screw, which is rotated in synchronization with the developing roller, is controlled by controlling the rotation speed of a development motor via a second motor driver, which is performed by a controller. In addition, the developer feeding speed of the first developer feeding screw is controlled by controlling the rotation speed of a first feeding motor via a first motor driver, which is also performed by the controller.
Further, there is a proposal in which when the revolution of the first feeding screw is R1 (rps) and the revolution of the second feeding screw is R2 (rps), the output from a toner concentration sensor is periodically read in a cycle of not less than a least common of 1/R1 and 1/R2.
Furthermore, there is a proposal for an image forming apparatus having various processing speeds and performing inductance detection to detect the developer concentration. In the developing device, the concentration detection is carried out only when the linear velocity of a screw for agitating and feeding developer is a predetermined first rotating velocity.
In these image forming apparatuses, the rotation speeds of the first and second feeding screws 102 and 103 depend on the rotation speed of the developing sleeve 101. Therefore, the screws feed the developer at a speed corresponding to the selected mode. Namely, the developer feeding speed is changed depending on the modes. Therefore, even when the developer feeding speed is changed due to change of the mode, the speed at which the developing sleeve 101 uses the developer is also changed depending on the mode, and a proper amount of developer is generally supplied to the developing sleeve 101.
However, as a result of an experiment of the present inventors, there is a case in which when the feeding speed of a developer is changed, the output from the magnetic permeability sensor is changed even though the toner concentration of the developer is not changed. The result of the experiment is illustrated in FIG. 5. FIG. 5 is a graph showing a relation between the revolution of the feeding screw 102 and the output from the magnetic permeability sensor.
In the experiment, the toner concentration of the developer is controlled to 7% by weight. It can be understood from FIG. 5 that when the revolution of the first feeding screw 102 changes, the output from the magnetic permeability sensor changes.
It is also confirmed from this experiment that since the output from the magnetic permeability sensor changes even when the toner concentration is not changed, the toner concentration determining operation has a large margin of error.
When the toner concentration determining operation has a large margin of error, the toner concentration controlling operation is performed based on the incorrect toner concentration, which is largely different from the correct toner concentration, a problem in that the developer has too high or too low toner concentration is caused.
In attempting to solve the problem, there is a proposal in which the developer feeding direction and speed at the magnetic permeability detecting part are set so as to be constant independently of the image forming modes (such as print speed oriented mode, image quality oriented mode, and the like) by maintaining the revolution of the feeding screw feeding the developer at the magnetic permeability detecting part so as to be constant independently of the revolution of the developing roller.
By using this technique, change of the output from a magnetic permeability sensor due to change of the print modes can be reduced. However, the technique has a drawback such that at a low image forming speed, the developer is not well-balanced in the developer container, thereby changing the developer drawing conditions of the developing roller.
For these reasons, the inventors recognized that there is a need for a developing device in which the toner concentration of the developer can be determined by a magnetic permeability sensor without affected by change of the process linear velocity of the developing device and in which the developer is fed while well balanced.